The present invention relates to a heat-sealable laminated material and to a method for the manufacture of such a laminate.
It is known that in the technique of packaging molecular-oriented polyester films or packages molecular-oriented polyester material are used, especially in cases where the packing material is expected to withstand great tensile stresses without being changed in shape. Polyester material has been used, for example, in connection with plastic bottles for pressurized contents such as e.g. refreshing beverages. The bottles are blown from a polyester material in such a manner that the inflation is carried out at a temperature which is such that the walls in the plastic bottle formed are molecular-oriented.
This molecular orientation substantially improves the tensile characteristics of the plastic material in spite of the package wall in the bottle being thinned out in connection with the blowing operation. The plastic bottles for refreshing beverages are generally provided with a screw cap or some other closing device, which is not directly sealed to the plastic material by surface fusion.
On stretching the polyester material, beside a thinning of the material, a conversion of the molecular structure takes place, so that the material becomes mainly crystalline. A polyester material which has been molecular-oriented in such a manner by stretching, is very difficult to heat-seal, and the heat-sealing characteristics become worse the more the material is stretched. In other words, the more the crystalline structure predominates in the material.
For the abovementioned reasons, thin film material of orientation-stretched polyester material has not found any appreciable application within the technique of packaging, since the material cannot be heat-sealed. In view of the excellent tensile strength of the material it would be desirable, however, to use orientation-stretched polyester material in many fields, provided it could be made heat-sealable. Experiments have been carried out using glue or so-called hot melt which can be sealed at a relatively low temperature instead of heat-sealing the material. Experiments have also been carried out coating orientation-stretched polyester material either before or after the orientation-stretching, with a plastic material of a relatively low melting point, e.g. polythene. However, it has been found that there are great difficulties in achieving good adhesion between the polythene layer and the polyester layer, and consequently the method has not found much application. In recent years, however, certain modified polyester materials have come on the market, so-called cyclohexane-modified polyesters or PETG, which polyester material may be subjected to a stretching at which ordinary polyester material is molecular-oriented, while the modified polyester material is not altered in its molecular structure or does not become crystalline to any great extent. The modification of the polyester material may be pushed to different lengths, which means that within certain limits the tendency towards crystallization can be controlled. The fact that the material fails to be molecular-oriented and to be crystallized means of course that the cyclohexane-modified polyester material to some extent is only reduced during the stretching operation but is not given the greatly improved tensile strength characteristics which on orientation-stretching are imparted to an ordinary polyester material. On the other hand, however, the heat-sealing characteristics of the stretched modified polyester material are maintained. The heat sealing characteristics are of decisive importance for the manufacture of packing containers from webs or sheets of packing material, which must be heat-sealed in order to create the closed space which is to hold the contents.
Neither the ordinary polyester material made very strong by molecular orientation nor the modified sealable polyester material is suitable by itself to be used as packing material, since the one material is strong but not sealable and the other material is sealable but not strong. They can advantageously be included jointly in the packing laminate, however, since the two materials, which are both polyester material, can be joined together very easily by surface fusion, either by extrusion coating of the one material onto the other, by separate extrusion of the two materials or by direct combination or else by so-called co-extrusion, where the two materials are extruded simultaneously, and in one layer, through one extruder die.
The disadvantages in connection with the application of orientation-stretched polyester material mentioned earlier can thus be overcome by the present invention which relates to a laminated material comprising at least one layer of a monoaxially or biaxially orientation-stretched polyester material of predominantly crystalline molecular structure, and at least one layer of a cyclohexanemodified polyester material, so-called PETG material, of predominantly amorphous molecular structure,. The layer of polyester material of predominantly amorphous molecular structure is substantially thinner than the layer of crystalline molecular structure.
The invention relates moreover to a method for the manufacture of such a laminate. The method is characterized in that two or more layers of polyester material are joined together by lamination or extrusion, e.g. co-extrusion, the one polyester material being modified and being of the type which can be subjected to a stretching treatment at a temperature below 100.degree. C. without the amorphous molecular structure being wholly lost, e.g. a polyester material of the type which is marketed under the designation PETG. The second layer is constituted of polyester material whose molecular structure becomes crystalline when the material is subjected to stretching at a temperature below 100.degree. C. The layers so combined are subjected to a joint stretching operation at a temperature below 100.degree. C.